This invention relates to high frequency oscillators using coaxial cavities and, more particularly, to such oscillators used in aircraft transponders.
In typical prior art pencil tube UHF oscillators, the feedback coupler consists of a wire probe coupled from the cathode line of the input cavity circuit through an aperture in the common wall to the output cavity circuit. This feedback technique does not give sufficient coupling to drive the tube to the tube's maximum RF capability. Another technique uses a rectangular strap probe in place of the wire probe with the widest part of the strap paralleling the center conductor of the output cavity over a sufficient length of the transmission line. This strap with sufficient length and with proper spacing from the center conductor provides enough feedback energy from the output cavity circuit to drive the tube to the maximum RF capability. This feedback circuit, however, must be rigid to withstand rigid vibration specifications. A typical vibration specification for such oscillators in aircraft transponders requires that they be subjected to vibration of constant total excursion of 0.10 inch from 5 to 55 Hz with a maximum acceleration of 1.5G and a constant acceleration of 1.0G from 55 to 2,000 Hz. Other vibrational tests will require the 55 to 2,000 Hz constant acceleration to a maximum of 10G's. The oscillator output frequency must stay within plus and minus 3 MHz of the center frequency and the power output must not drop below a minimum RF power output. Further, in a mass production environment, it is desirable to achieve an adjustment of the coupling circuit of at least 2 to 1 coupling ratio after the oscillator assembly has been completed. This adjustment corrects for operational tolerances such as tubes, circuits, etc. The rectangular strap pick-up is in the form of a cantilever beam structure and is subject to resonances that could fall within the forbidden vibration frequency band. By increasing the thickness of the strap to move the feedback structure resonant frequency up out of the band of interest also increases rigidity which decreases the ease of adjusting the feedback coupling.